Add a tiny procfs helper to splice a dentry that cannot be mounted upon.

Link: https://lore.kernel.org/r/20240806-work-procfs-v1-3-fb04e1d09f0c@kernel.orgReviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>